The present invention relates to polyimide polymers having superior electrical and mechanical properties which are the imidized condensation product of 2,2-bis (3-amino-phenyl)hexafluoropropane, (referred to as (3,3'-6F diamine) or 2,2-bis(4-amino-phenyl)hexafluoro propane (referred to as 4,4'-6 F diamine) and at least one other diamine, with pyromellitic dianhydride and one or more additional aromatic dianhydrides, and to articles prepared therefrom.
Polyimide condensation products of 4,4'-6 F diamine and aromatic dianhydrides are known in the art and described in U.S. Pat. No. 3,356,648. These polyimides are produced by first and dianhydride, such as pyromellitic dianhydride and 4,4'-6 F diamine, in suitable solvent, followed by conversion of the polyamic acid to the polyimide. Imidization is preferably carried out by the application of a solution of the polyamic acid to a substrate, by the evaporation of the solvent and by heating the resultant film to temperatures in the order of 270 to 300.degree. C. for about 2 hours. These polyimides possess excellent heat stability, good electrical properties and high tensile strength.
U.S. Pat. No. 4,592,925 discloses similar polyimides prepared by imidization of the polyamic condensation product of 3,3'-6F diamine with 4,4'-hexafluoro isopropylidenebis(phthalic anhydride). These polyimides are disclosed to be particularly suitable as protective coatings for solar cells.
While polyimides based on the reaction product of pyromellitic dianhydride and 6F-diamine offer good heat stability, mechanical and electrical properties, these polyimides are generally insoluble in organic solvents such as N-methyl pyrrolidone (NMP), acetone and the like, and produce very brittle films. It has been suggested in the prior art the solubility in halogenated solvents of copolyetherimides based on the condensation product of 2,2-bis-[4,(2,3-dicarboxyphenoxy)phenyl] propane dianhydride and a diamine such as m-phenylene diamine can be enhanced by the inclusion of 2,2-bis(3,4 dicarboxyphenyl)hexafluoropropane dianhydride in the reaction mixture to form a copolyetherimide. In addition, U.S. Pat. No. 3,424,718 teaches that the adhesive qualities of polyimide polymers are enhanced by including a mixture of aromatic and aryl aliphatic diamines in the reaction mixture. In one example, the polyimide is prepared by reacting two different dianhydrides, namely pyromellitic and 2,2-bis(3,4 dicarboxyphenyl)hexafluoropropane and two different diamines, namely m-xylene diamine and bis(4-aminophenyl) ether.
While it is desirable to impart solvent solubility to solvent insoluble polyimides to render them useful in applications where the polyimide is applied to a substrate in the form of a solution, or in the formation of composites, it is also desirable that the mechanical and insulating properties of such polyimides not be compromised.